Conventionally, internal gear oil pumps, which are generally compact and simply constructed, are widely used as pumps for lubrication oil in automobiles and as oil pumps for automatic transmissions, etc. Such an oil pump comprises an inner rotor formed with “n” external teeth (hereinafter “n” is a natural number), an outer rotor formed with “n+1” internal teeth which are engageable with the external teeth, and a casing in which a suction port for drawing fluid and a discharge port for discharging fluid are formed, and fluid is drawn and is discharged by rotation of the inner rotor which produces changes in the volumes of cells formed between the inner and outer rotors.
With regard to such internal gear oil pumps, in order to reduce pump noise and to increase mechanical efficiency, various technical means have been employed such as setting a tip clearance having appropriate size between the tooth tips of the inner and outer rotors, modifying tooth profiles which are formed using, for example, cycloid curves, etc. More specifically, in some oil pumps, the profiles of the teeth of the outer rotor are uniformly cut so as to ensure a clearance between the surfaces of the teeth of the inner and outer rotors, or alternatively, the cycloid curve defining the shape of the teeth are partially flattened so as to modify the tooth profiles (see, for example, Patent Document 1)
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 05-256268.
However, in such conventional means of setting a tip clearance by uniformly cutting the profiles of the teeth, or flattening the cycloid curve by adjusting the diameter of a rolling circle that generates the cycloid curve or by forming a portion of the tooth profile using a straight line, even though a sufficient tip clearance may be obtained, a clearance between the tooth surfaces is also increased, which leads to problems such as increase in transmission torque loss due to play between the rotors or due to slippage between the tooth surfaces, pump noise due to impacts between the rotors, etc.
Moreover, when inappropriate clearance is provided between the tooth surfaces by the adjustment of tooth surface profiles, hydraulic pulsation may be produced or increased, which may lead to problems such as decrease in pumping performance or mechanical efficiency, pump noise, etc.